DE4243559A1 - Process for the purification of a technical iron chloride solution by selective precipitation - Google Patents

Description

The invention is directed to a method of removal sewage-polluting metal ions from an essentially Acidic solution containing iron (II) chloride in diluted Cyclonic dust from the hydrochloric acid Titanium dioxide production after the chloride process by lifting the pH and separating the metal hydroxides formed.

As a precipitant and flocculant in wastewater treatment iron (II) chloride solutions are used (water technology Information from KRONOS TITAN-GMBH, Ferrofloc®). Iron (II) chloride falls in large quantities in the Titanium dioxide production using the so-called chloride process at. This is a raw material containing titanium and iron, e.g. B. titanium slag or ilmenite, in the presence of coke as Reducing agent at temperatures around 1000 ° C in one Fluidized bed reactor chlorinated. In addition to titanium tetrachloride Iron (II) chloride is also formed. From the the reactor leaving the iron (II) chloride together with the water-insoluble solids - essentially coke, not digested titanium dioxide and silicon dioxide - as well other metal chlorides in a downstream cyclone deposited. The separated mixture is called cyclone dust designated. The elaborate processing of the cyclone dust is for example, described in US 3,867,515.

By increasing the cyclone dust in dilute hydrochloric acid and Removing the water-insoluble constituents becomes a solution obtained, mostly iron (II) chloride, but also Aluminum chloride, manganese chloride, magnesium chloride, Zirconium chloride and trace elements such as chrome, niobium and Contains vanadium as chloride. If such Iron (II) chloride solution for wastewater treatment and  Sludge conditioning should be used be that because of the raw material composition inevitable proportions of trace elements over the Chemical does not get ingredients into the wastewater in turn raise disposal problems. The "burdened" Iron (II) chloride solution must therefore continue before it can be used be cleaned, d. H. especially the chrome, niobium and Vanadium ions must be removed from the solution.

It is known that when the pH of such a solution is raised above 1, the dissolved metal ions accordingly their solubility products precipitate as hydroxides; Iron (II) ions remain up to a pH of over 6 in Solution. Thus, through selective precipitation, theoretically a contaminated ("technical") iron chloride solution in a clean iron (II) chloride solution are transferred. In the In practice, however, this way is not feasible. One admits an acidic solution from cyclone dust is a neutralizing agent such as sodium hydroxide, calcium hydroxide, Magnesium hydroxide, sodium carbonate or calcium carbonate, then metal hydroxide sols are formed, which because of their gel-like Structure extremely difficult to filter and not suitable for landfill are.

In the Russian patent specification 676 559 the precipitation of Metal hydroxides from a solution that in addition to iron contains unwanted heavy metal ions by adding 8% sodium bicarbonate solution described at 20 ° C; by raising the pH to 4.5 in particular Chromium can be accumulated in the filter cake. Among the Precipitation conditions given are the gel-like hydroxides not filterable with reasonable effort. About the The nature of the filter cake is described in the patent no information given.

Another way of removing chrome from Metal chloride solutions are described in US 4,765,908. Bentonites, especially montmorillonite, are  at least one flocculant and alkali or Alkaline earth carbonates together with zirconium catalysts and used at least one polyelectrolyte, whereby the Heavy metal ions are bound adsorptively and the Separation of the filter cake presents no difficulties. The filter residue is also landfillable. However, it is Processes very complex due to the use of chemicals, and there is also an above average size Volume of sludge.

In German patent application P 41 30 808.5 suggested removing the unwanted ions from it an iron (II) chloride solution made from cyclone dust by selective crystallization. That there described method leads to the desired Cleaning the iron (II) chloride solution, however Investment and operating costs are quite right with this procedure considerably.

The invention is based on the problem of removing undesired metal ions, in particular chromium, niobium, zirconium and vanadium from an iron (II) chloride solution, amounts of around 15 m ³ / h having to be managed and one usually occurring when the undesired metal ions precipitate as hydroxides Extremely finely divided gel-like suspension is formed, the precipitated metal hydroxides have so far not been able to be separated off sufficiently quickly and completely by filtration and the filter residue in the previous form is not suitable for landfill.

It has now been found that through a special two-stage Process control of partial neutralization of metal hydroxides ions to be removed, which are easier than before in to be separated on an industrial scale and thus for the first time cleaning large quantities of contaminated Iron (II) chloride solutions by selective precipitation becomes economically possible. The procedure is thereby characterized in that the solution initially with a first  Neutralizing agent to a pH between 0.3 and 0.8 is adjusted and then the blunted solution in a Submission with a second neutralizing agent in water or purified iron (II) chloride solution is dosed so that the pH there never falls below 2.0 and that second neutralizing agent poorly soluble and in excess based on the waste water-polluting metal ions is available.

Those obtained by the process according to the invention Precipitation products are very easy to filter; the filter cake is landfillable.

The second has proven to be particularly advantageous Neutralizing agent proven calcium carbonate. It stands unlimited available and it seems just because of it Poor solubility in water or an iron chloride solution, d. H. through a kind of natural buffering effect of the suspension in the template, to a less spontaneous nucleation and thus avoiding a gel state. Of the solid formed with this neutralizing agent filterable much better. With this procedure nucleation appears to be reduced. The formation of Iron (II) hydroxide is due to the low pH in the Submission not possible. Formation of iron (III) hydroxide takes place due to the inherent carbon dioxide development (Protective gas atmosphere) also does not take place; an oxidation with the atmospheric oxygen is excluded.

While very special as a second neutralizing agent Calcium carbonate is preferred, which in a suspension shows the desired buffer effect, comes first Neutralizing agent for the dulling of the technical Iron chloride solutions in addition to alkaline earth carbonate - again preferably calcium carbonate or dolomite stone flour - preferred dulling by iron or iron compounds in question. The addition of iron increases the amount of the desired End product without introducing by-products  become. When adding scrap iron first Neutralizing agents are not special measures note. When using sinter, a mixed oxide of two and trivalent iron, however, iron (III) ions are formed when introducing the solution into the second neutralizing agent would partially precipitate as iron (III) hydroxide and so with the filter residue will be lost. Therefore, when used of sinter as the first neutralizing agent before the blunted solution in the template with the second Neutralizing agent is given using iron (III) ions Scrap or other reducing agent in iron (II) ions being transformed.

Calcium carbonate is also used first Neutralizing agent, contains the iron (II) chloride solution relatively high amounts of calcium chloride. Because that is often undesirable is from such a solution by adding Iron sulfate the calcium sulfate precipitated and separated be, the concentration of iron (II) chloride increases accordingly.

Preferably, the temperature of the suspension in the template 60 to 90 ° C; be. Under these conditions form Precipitation products that are separated and opened particularly cheaply Landfill can be placed.

It turned out to be cheap, the second To stretch the partial neutralization step. The Dosing with the first neutralizing agent blunted solution in the template should be preferred so take place that per kilogram of calcium carbonate in the template 25th up to 33 l of blunted solution can be added per hour.

The invention is further exemplified below described.  

A typical "contaminated" iron (II) chloride solution Cyclone dust in dilute hydrochloric acid has the following Composition:

With this technical iron (II) chloride the 4th following experiments.

Example 1 Dulling and precipitation only with calcium carbonate (Limestone flour)

1200 ml of the contaminated iron (II) chloride solution are heated to 70 ° C. 41.5 g of limestone powder (approx. 97% CaCO ₃ , grain size less than 0.1 mm) are stirred in within 30 minutes. After adding the first neutralizing agent, the pH is 0.78.

From this with the first neutralizing agent "blunted" solution are 30 ml per minute in a submitted limestone powder suspension (72 g limestone powder in 72 g of water) at 70 ° C. After the addition, the pH Value in the template 2.8. The mixture is a further 30 min touched; the pH increases to 3.4. The 70 ° C hot suspension is filtered. In all examples the Filtration carried out at 50 mbar and a Nutsche with a diameter of 13 cm is used. The filtration process is given in Table 1.  

Table 1

The filter cake is 10 mm thick and puncture-proof. The clear Filtrate has the following composition:

The chromium to iron ratio has increased from 0.0133 0.000088, the vanadium to iron ratio of 0.0274 0.000033 improved. In relatively short times (good Filterability), a depletion of about 97% of the polluting ions reached, whereby from the Precipitation products a puncture-resistant filter cake is obtained.

Example 2 Dulling the contaminated iron chloride solution with roll sintering and reducing the Fe ³⁺

1200 g of the above-mentioned contaminated (technical) iron (II) chloride solution are stirred with 25.5 g of roll sinter (FeO = 68%; Fe ₂ O ₃ = 32%) at 80 ° C for 1 hour. To reduce the Fe (III) ions, iron powder is added and the mixture is stirred at 80 ° C. for a further 20 min. The pH is 0.75. This blunted solution is added to a template at a rate of 30 ml / min.

It again contains 72 g of limestone flour in 72 g of water and has a temperature of 70 ° C. At the end of the dosage, after 30 min, the pH value 2.8 is reached. After 30 minutes With further stirring, the pH rises to 3.3. The Filtration at 70 ° C takes place on the suction filter under the same Conditions as in example 1. The filtration process is in Table 2 indicated.

Table 2

The composition of the solution is:

The chromium-iron ratio has increased from 0.0133 to 0.000058, the vanadium-iron ratio from 0.0274 to 0.000039 improved.

Example 3 Influence of dosing speed

The procedure is as in Example 2, but the Dosing speed of the loaded solution into the presented Calcium carbonate suspension raised to 50 ml / min. The pH The value at the end of the addition is 2.2 and after 30 min Stirring time at 3.2. As can be seen in Table 3, the filterability does not deteriorate, but the Depletion of chrome to 0.00018 and vanadium to 0.0027 decreased.  

Table 3

The filtrate has the following composition:

Example 4

Conventional neutralization as a comparative example. 1200 g of the acidic FeCl ₂ solution were heated to 80 ° C. and blunted as in Example 2 with roll sinter and trivalent iron ions were reduced with iron. This solution (pH = 0.78; 70 ° C) is now (partially) neutralized by adding 72 g of solid limestone powder in 30 min.

At the end of the addition, the pH rose to 2.7 and after a further stirring time of 30 minutes, a value of 3.0 is reached. It is the 70 ° C warm suspension as described above filtered. Table 4 shows the essential deteriorated filterability.

Table 4

The filtration comes to a standstill relatively soon. The approx 3 mm thick filter cake consists of a gel-like,  semi-fluid mass. The filtrate has the following Composition:

Only the depletion is correspondingly small (to 0.00037 at Cr / Fe and 0.00061 at V / Fe).

Claims (7)

1. A process for removing wastewater-polluting metal ions from an essentially iron (II) chloride-containing acidic solution of cyclone dust dissolved in dilute hydrochloric acid from titanium dioxide production after the chloride process by raising the pH and separating off the metal hydroxides formed, characterized in that the solution initially is adjusted to a pH between 0.3 and 0.8 with a first neutralizing agent and then the solution thus blunted is dosed into a template with a second neutralizing agent in water or purified iron chloride solution so that the pH there is 2 , 0 never falls below and the second neutralizing agent is sparingly soluble and is present in excess based on the waste water-polluting metal ions. 2. The method according to claim 1, characterized in that the second neutralizing agent is an alkaline earth carbonate, preferably calcium carbonate or dolomite stone. 3. The method according to claim 1 or 2, characterized in that the amount of the second neutralizing agent up to 1.4 times the stoichiometric amount waste water-polluting metal ions. 4. The method according to any one of claims 1 to 3, characterized characterized in that the first neutralizing agent Alkaline earth carbonate or scrap iron. 5. The method according to claim 1 to 3, characterized in that the first neutralizing agent is iron sinter and by adding a reducing agent, preferably iron, before initiating the solution in the template Iron (III) ions can be reduced.   6. The method according to any one of claims 1 to 5, characterized characterized in that the temperature of the suspension in the Template is 60 to 90 ° C. 7. The method according to any one of claims 1 to 6, characterized characterized in that the metering speed of the blunted solution in a presented calcium carbonate Suspension 25 l / h to 33 l / h, based on 1 kg Calcium carbonate in the template.